Making energy out of waste heat with simple ingredients

Differences as small as 20°C can be turned into useful power.

The world is awash in our waste heat. Our computers, our motors, our electrical generating plants—all of them shed heat into the environment. That's in part because there's no easy way to capture its energy and put it to use. All the existing methods we have for harvesting waste heat are either inefficient or uneconomical.

Now, some researchers have come up with a new method of grabbing some of that waste heat and potentially putting it to work. Their system relies on nothing more complex than water and a polymer membrane and, even in its first test form, it's already capturing roughly half of the possible Carnot efficiency available to the system.

We already generate lots of electricity via heat differences. It's just that those differences are large—large enough to create the pressure differences needed to drive turbines. Waste heat often becomes waste simply because the temperature differences are small, on the order of dozens of degrees Celsius, rather than hundreds.

To convert smaller temperature differences into useful electricity generally relies on thermoelectric materials. These are often expensive, frequently involving hard-to-get (and sometimes toxic) metals. So, while these could work, it's very hard to get them to do so economically.

The team behind the new work, which included researchers at Columbia, Vanderbilt, and Yale, decided to try a radically different approach. It's based on a simple principle: if you have two identical fluids at different temperatures, it's energetically favorable for molecules to evaporate out of the warm one and condense into the cooler one. Over time, this will lead to an increase in volume in the cooler liquid.

Now, obviously, this won't work if you just put two pots of water at different temperatures next to each other, because there's no reason for the evaporated water to go to the other pot when there's an entire room's worth of atmosphere to diffuse into. So the team relied on a carefully constructed container that kept the warm water (representing waste heat) separated from the cold water via a special membrane.

The membrane was constructed from polytetrafluoro ethylene, processed so that its typical pore size is only 20nm. The material is extremely hydrophobic, which means it repels water. The small pore size and hydrophobicity means that water's surface tension keeps it from entering the pores and transiting across the membrane. But water vapor doesn't have surface tension, so it'll happily cross.

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In this setup, water vapor from the warm reservoir evaporates, crosses the membrane, and condense into the cooler reservoir. Over time, this causes the cool reservoir to expand. Since the membrane isn't entirely flexible, this causes the pressure to rise. The authors suggest that it's relatively easy to use that pressure to do useful work, even driving a small generator.

This may seem like it would be an incredibly inefficient process. But the authors calculated that they could use a 5°C difference to generate up to 400 bars of pressure. And, when they built actual test versions, they were able to generate 3.5 Watts per square meter of membrane with a 40°C temperature difference.

They also spiked the water with some table salt to confirm that there was no fluid transfer across the membrane; there wasn't, indicating that the water was transiting as vapor, at least at lower pressures. If the pressure were allowed to get high enough, the membrane lost its integrity. Even at lower pressures, efficiency of the device tailed off a bit, as the pressure compressed the internal space in the membrane, limiting vapor movement.

But the authors say that at the sorts of pressures they'd expect their device to operate, it should be able to extract up to seven percent of the energy represented by the temperature difference. That may not sound like much, but it's 58 percent of the theoretical Carnot efficiency limit. The devices would be somewhat less efficient overall, since they'd also need pumps to keep the two liquids moving through the system.

While the authors worked with a liquid that's cheap and abundant (water), it might also be possible to use fluids with properties that are tailored to specific temperature differences, given the right membrane. And it should also be possible to construct membranes that hold up to pressure better, allowing the system to operate in ranges where extracting useful energy is a bit more efficient.

Whether any of these will work out to provide a way of economically harvesting waste heat isn't clear at this point. But what is clear is that this is a radically different approach to harvesting waste heat than most of the things that are currently under study.